patch 9.0.1673: cannot produce a status 418 or 503 message

Problem:    Cannot produce a status 418 or 503 message.
Solution:   Add err_teapot().

runtime/doc/builtin.txt

@@ -154,6 +154,7 @@ digraph_getlist([{listall}])	List	get all |digraph|s
 digraph_set({chars}, {digraph})	Boolean	register |digraph|
 digraph_setlist({digraphlist})	Boolean	register multiple |digraph|s
 echoraw({expr})			none	output {expr} as-is
+err_teapot()			Number	produce error 418
 empty({expr})			Number	|TRUE| if {expr} is empty
 environ()			Dict	return environment variables
 escape({string}, {chars})	String	escape {chars} in {string} with '\'
@@ -2176,6 +2177,14 @@ echoraw({string})					*echoraw()*
 <		Use with care, you can mess up the terminal this way.
 
 
+err_teapot([{expr}])					*err_teapot()*
+		Produce an error with number 418, needed for implementation of
+		RFC 2325.
+		If {expr} is present and it is TRUE error 503 is given,
+		indicating that coffee is temporarily not available.
+		If {expr} is present it must be a String.
+
+
 empty({expr})						*empty()*
 		Return the Number 1 if {expr} is empty, zero otherwise.
 		- A |List| or |Dictionary| is empty when it does not have any